1. Field of the Invention
The invention relates to a reflection type liquid crystal display and a method of fabricating the same.
2. Description of the Related Art
A reflection type liquid crystal display includes a light-reflection plate therein, and reflects external lights entering the liquid crystal display, at the light-reflection plate. The thus reflected external lights are used as a light source. Hence, a reflection type liquid crystal display does not need backlight as a light source. For this reason, a reflection type liquid crystal display is generally adopted as a display in a portable terminal device such as a pager. A reflection type liquid crystal display is superior to a transmission type liquid crystal display with respect to power consumption, thickness, and weight.
A reflection type liquid crystal display includes a light-reflection plate therein, and reflects external lights entering the liquid crystal display, at the light-reflection plate. The thus reflected external light is used as a light source. Hence, a reflection type liquid crystal display does not need backlight as a light source. For this reason, a reflection type liquid crystal display is generally adopted as a display in a portable terminal device such as a pager. A reflection type liquid crystal display is superior to a transmission type liquid crystal display with respect to power consumption, thickness, and weight.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional reflection type liquid crystal display.
The illustrated conventional liquid crystal display 10 is comprised of a lower substance 1A, an upper substrate 1B facing the lower substrate 1A, and a liquid crystal layer 14 sandwiched between the lower and upper substrate 1A and 1B. For instance, the liquid crystal layer 14 contains GH liquid crystal. In this specification, the upper substrate 1B is a substrate through which external lights pass and on which characters such as figures and letters are displayed by reflected external lights, and the lower substrate 1A is a substrate at which external lights having passed through the upper substrate 1B are reflected.
The lower substrate 1A is comprised of a glass substrate 2A, a thin film transistor 6 acting as an active matrix drive device, formed on the glass substrate 2A, and having a reverse stagger structure, an interlayer insulating film 11 formed over the thin film transistor 6 and composed of polyimide, and a light-reflective electrode plate 24 in electrical connection with a source electrode 21 or a drain electrode 22 of the thin film transistor 6, acting as a light-reflective plate and a pixel electrode.
The upper electrode 1B is comprised of a glass substrate 2B, a color filter 3 formed on the glass substrate 2B, and a transparent electrode 4 formed over the color filter 3.
In the illustrated reflection type liquid crystal display 10, external light 15 pass through the glass substrate 2B, the color filter 3, the transparent electrode 4, and the liquid crystal layer 14, and are reflected at the light-reflective electrode plate 24. The reflection type liquid crystal display 10 uses the thus reflected light 16 as a light source.
The reflection type liquid crystal display 10 is required to present bright and white display when a light is transmitted through the liquid crystal layer 14. To meet with this requirement, it is necessary for the reflection type liquid crystal display 10 to reflect the external light 15 having passed through the upper electrode 1B in various directions, outwardly through the upper electrode 1B. Hence, the polyimide film 11 is designed to have raised and recessed portions at a surface thereof to thereby form similar raised and recessed portions at a surface of the light-reflective electrode plate 24 formed on the polyimide film 11. A shape of the raised and recessed portions of the polyimide film 11 is a key for determining display performance of the reflection type liquid crystal display 10.
A method of fabricating the reflection type liquid crystal display 10 is explained hereinbelow with reference to FIGS. 2A to 2H. In FIGS. 2A to 2H, the term xe2x80x9cPRxe2x80x9d means that photolithography is carried out in that step, and a figure at the head of the term xe2x80x9cPRxe2x80x9d means a total number of photolithography steps. For instance, xe2x80x9c2PRxe2x80x9d means that the second photolithography is carried out in the step illustrated in an associated Figure.
First, as illustrated in FIG. 2A, a gate electrode 17 is formed on a glass substrate 2.
Then, as illustrated in FIG. 2B, an insulating film 18, a semiconductor layer 19, and a doped layer 20 are formed in this order over the glass substrate 2.
Then, as illustrated in FIG. 2C, the semiconductor layer 19 and the doped layer 20 are patterned to thereby form an island on the insulating film 18.
Then, as illustrated in FIG. 2D, a source electrode 21 and a drain electrode 22 are formed around the island to thereby fabricate a transistor.
Then, as illustrated in FIG. 2E, an insulating film 18 is deposited entirely over the product.
Then, an organic insulating film 25 is deposited entirely over the insulating film 18, and is patterned so as to form raised and recessed portions 26 above a region where a light-reflective plate is to be formed, as illustrated in FIG. 2F.
Then, as illustrated in FIG. 2G, there is formed a contact hole 23 reaching the source electrode 21 through the organic insulating film 25 and the insulating film 18.
Then, as illustrated in FIG. 2H, the light-reflective electrode plate 24 is formed over the organic insulating film 25 so that the contact hole 23 is filled with the material of which the reflection type liquid crystal display 10 is composed.
As shown in FIGS. 2A, 2C, 2D, 2F, 2G and 2H, photolithography is carried out six times in the method.
The method having been explained with reference to FIGS. 2A to 2H is suggested in Japanese Patent Publication No. 61-6390, and by T. Koizumi and T. Uchida in Proceedings of the SID, Vol. 29, pp. 157, 1988.
In order to fabricate a reflection type liquid crystal display capable of presenting bright and high-grade display, it has been conventionally necessary to form a high-performance switching device and a high-performance reflective plate on a common insulating substrate. The formation of them on a common substrate requires carrying out a lot of steps for film deposition, photolithography steps, and etching steps. Accordingly, the conventional method of fabricating a reflection type liquid crystal display cannot avoid including a lot of fabrication steps, taking much time, and needing much cost.
Japanese Unexamined Patent Publication No. 6-75238 has suggested a method of fabricating a reflection type liquid crystal display, including the steps of fabricating a thin film transistor on a substrate, forming an organic insulating film entirely over the substrate and the transistor, concurrently forming a contact hole, and a raised portion, and forming a reflective electrode over the contact hole and the raised portion. A drain electrode of the transistor is electrically connected to the reflective electrode through the contact hole.
Japanese Unexamined Patent Publication No. 6-273800 has suggested a reflection type liquid crystal display. In the suggested reflection type liquid crystal display, a gate bus, a source bus, and a thin film transistor are formed on a substrate. An organic insulating film having raised portions is formed over the substrate, covering the thin film transistor therewith. A light-impermeable film is formed on the organic insulating film above the thin film transistor, and a reflective electrode is formed above a region other than the thin film transistor. A gap is formed between the reflective electrode and the light-impermeable film. An alignment film is formed over the product. A liquid crystal layer is sandwiched between the substrate and another substrate on which a color filter, an electrode, and an alignment film are formed.
Japanese Unexamined Patent Publication No. 6-342153 has suggested a reflection type liquid crystal display. In the suggested reflection type liquid crystal display, an insulating film to be formed between a reflective electrode and a thin film transistor is designed to have light-absorptive characteristic and/or light-scattering characteristic. Thus, it is no longer necessary to use a black mask which has been absolutely necessary for fabricating a reflection type liquid crystal display.
Japanese Unexamined Patent Publication No. 9-54318 has suggested a method of fabricating a reflection type liquid crystal display. The method includes the steps of forming a thin film transistor on an insulating substrate, forming an insulating film over the insulating substrate, and forming a light-reflective plate over the insulating film so that the light-reflective plate is electrically connected to the thin film transistor. The light-reflective plate is designed to have raised and recessed portions at a surface thereof. Those raised and recessed portions are formed at the same time when a gate electrode, a gate insulating film, and a semiconductor layer are etched for forming an island.
It is an object of the present invention to provide a reflection type liquid crystal display which is capable of providing high brightness and high quality display in the decreased number of fabrication steps.
It is also an object of the present invention to provide a method of fabricating such a reflection type liquid crystal display.
In one aspect of the present invention, there is provided a reflection type liquid crystal display including (a) a first substrate, (b) a second substrate having a transparent electrode formed thereon in facing relation to the first substrate, and (c) a liquid crystal layer sandwiched between the first and second substrates, the first substrate including (a1) an insulating substrate, (a2) a switching device fabricated on the insulating substrate, (a3) at least one projection projecting towards the second substrate and formed on the insulating substrate in a line with the switching device, (a4) a first insulating film covering both the switching device and the projection therewith and having first raised and recessed portions formed in accordance with height of the switching device and the projection, and (a5) a light-reflective plate formed over the first insulating film and having second raised and recessed portions formed over the first raised and recessed portions, the switching device being in electrical connection with the light-reflective plate.
It is preferable that the switching device has the same height as a height of the projection.
It is also preferable that the first insulating film has a portion on the projection, the portion having a cross-sectional area decreasing towards the second substrate.
The first insulating film may be composed of inorganic or organic material. However, it is preferable that the first insulating film is composed of photosensitive material. By composing the first insulating film of photosensitive material, it is possible to pattern the first insulating film directly by a step of exposing the first insulating film to a light and a step of developing the same. Hence, a photoresist step, an etching step, and a photoresist removal step, which were necessary to carry out in a conventional method of fabricating a reflection type liquid crystal display, can be all omitted, resulting in reduction in the number of fabrication steps.
It is preferable that the light-reflective plate is formed only above the projection. The light-reflective plate is generally composed of metal having a high reflectance ratio.
It is preferable that the projection is formed to be pillar-shaped, in which case, the projection is preferably spaced away from adjacent projection by a distance in the range of 3 xcexcm to 20 xcexcm both inclusive in a plane parallel to the first substrate. As an alternative, the projection may be formed to be strip-shaped.
It is preferable that the projection has a height in the range of 0.4 xcexcm to 4 xcexcm both inclusive. It is also preferable that the first insulating film has a thickness in the range of 0.6 xcexcm to 4 xcexcm both inclusive.
For instance, the switching device may be comprised of a thin film transistor, in which case, the transistor may be of forward stagger type or of reverse stagger type. As an alternative, the switching device may be comprised of a metal/insulator/metal (MIM) diode.
There is further provided a reflection type liquid crystal display including (a) a first substrate, (b) a second substrate having a transparent electrode formed thereon in facing relation to the first substrate, and (c) a liquid crystal layer sandwiched between the first and second substrates, the first substrate including (a1) an insulating substrate, (a2) a switching device fabricated on the insulating substrate, (a3) at least one projection projecting towards the second substrate and formed on the insulating substrate in a line with the switching device, (a4) a first insulating film formed on top of both the switching device and the projection, (a5) a second insulating film covering both the switching device and the projection therewith and having first raised and recessed portions formed in accordance with height of the switching device and the projection, and (a6) a light-reflective plate formed over the second insulating film and having second raised and recessed portions formed over the first raised and recessed portions, the switching device being in electrical connection with the light-reflective plate.
It is preferable that the first insulating film is tapered at a sidewall thereof, or rounded at a top thereof.
It is preferable that the switching device includes a third insulating film composed of the same material as that of the first insulating film, and that the projection has a multi-layered structure including at least one of a metal film, a fourth insulating film, and a semiconductor film, and the first insulating film formed thereon. The one of a metal film, a fourth insulating film, and a semiconductor film is composed of the same material as a material of which a layer constituting the switching device is composed.
The first insulating film may be composed of inorganic or organic material. However, it is preferable that the first insulating film is composed of photosensitive material. By composing the first insulating film of photosensitive material, it is possible to pattern the first insulating film directly by a step of exposing the first insulating film to a light and a step of developing the same. Hence, a photoresist step, an etching step, and a photoresist removal step, which were necessary to carry out in a conventional method of fabricating a reflection type liquid crystal display, can be all omitted, resulting in reduction in the number of fabrication steps.
It is preferable that the first insulating film is composed of thermally melting or contracting material.
In another aspect of the present invention, there is provided a method of fabricating a reflection type liquid crystal display, including the steps of (a) forming at least one of a metal film, a first insulating film, and a semiconductor film on an insulating substrate, (b) patterning the at least one of a metal film, a first insulating film, and a semiconductor film to thereby form a switching device and at least one projection on the insulating substrate, (c) forming a second insulating film covering both the switching device and the projection therewith and having first raised and recessed portions formed in accordance with height of the switching device and the projection, and (d) forming a light-reflective plate over the second insulating film, the light-reflective plate having second raised and recessed portions formed over the first raised and recessed portions.
It is preferable that the switching device is formed in the step (b) so that it has the same height as a height of the projection.
It is preferable that the second insulating film is formed in the step (c) so that a portion thereof on the projection has a cross-sectional area decreasing towards the second substrate.
The light-reflective plate may be formed preferably only above the projection in the step (d).
The projection may be formed in the step (b) to be pillar-shaped or strip-shaped.
There is further provided a method of fabricating a reflection type liquid crystal display, including the steps of (a) forming at least one of a metal film, a first insulating film, and a semiconductor film on an insulating substrate, (b) forming a second insulating film over the at least one of a metal film, a first insulating film, and a semiconductor film, (c) patterning both the second insulating film and the at least one of a metal film, a first insulating film, and a semiconductor film to thereby form a switching device and at least one projection on the insulating substrate, (d) forming a third insulating film covering both the switching device and the projection therewith and having first raised and recessed portions formed in accordance with height of the switching device and the projection, and (e) forming a light-reflective plate over the third insulating film, the light-reflective plate having second raised and recessed portions formed over the first raised and recessed portions.
It is preferable that the method further includes the step of tapering the second insulating film at a sidewall thereof. It is preferable that the method further includes the step of making the second insulating film rounded at a top thereof.
It is preferable that the switching device is formed in the step (c) so that it has the same height as a height of the projection.
The third insulating film may be formed in the step (d) so that a portion thereof on the projection has a cross-sectional area decreasing towards the second substrate.
The projection is comprised of either a film or films formed for fabricating an active matrix drive device or a transistor, or a combination such film or films and an insulating film acting as a mask to be used when any pattern for fabricating an active matrix drive device or a transistor is formed. The first (or second) insulating film is formed over the projection and the transistor so that the first (or second) insulating film has smooth raised and recessed portions. The light-reflective plate formed on the first (or second) insulating film would have raised and recessed portions which would reflect external lights in desired directions.
That is, it is possible to form a transistor and a projection in a common layer or common layers in a common step.
The projection may be formed pillar-shaped or strip-shaped. As a result, it is no longer necessary to form raised and recessed portions of different material in a separate step in a process for fabricating a transistor, ensuring reduction the number of fabrication steps.
The second raised and recessed portions of the light-reflective plate, on which display performance of a reflection type liquid crystal display is dependent, are dependent in shape on the projection or projections formed by a patterning step. Hence, the second raised and recessed portions can be controlled with respect to a shape, ensuring a reflection type liquid crystal display providing high brightness.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.